This invention relates to a drive device for an image display device including scan electrodes arranged in a matrix-like manner, and more particularly to a drive device suitable for use for an image display device including field emission cathodes.
Application of an electric field as high as 10.sup.9 V/m to a surface of a metal or semiconductor material leads to a tunnel effect which permits electrons to pass through a barrier, resulting in electrons being discharged to a vacuum atmosphere even at a normal temperature. In the art, such a phenomenon is referred to as "field emission" and a cathode which is adapted to emit electrons based on such a principle is referred to as "field emission cathode" (hereinafter also referred to as "FEC").
Recently, semiconductor processing techniques have permitted a field emission cathode of the surface discharge type to be formed of arrays of field emission cathodes of a size as small as microns, leading to research and development of an image display device which has such field emission cathodes incorporated therein.
Now, a so-called Spindt-type field emission cathode which is an example of a field emission cathode produced by such semiconductor processing techniques will be described hereinafter with reference to FIG. 3. The FEC includes a cathode electrode 100 made of a metal material such as aluminum or the like and formed on a substrate 102 of glass or the like by vapor deposition. The cathode electrode 100 is formed thereon with a plurality of emitters 104 of a conical shape each made of metal such as molybdenum or the like.
The cathode electrode 100 is formed on a portion thereof on which the emitters 104 are not arranged with a film 106 of silicon dioxide (SiO.sub.2), which is then formed thereon with a gate 108. The gate 108 and SiO.sub.2 film 106 are formed with a plurality of through-holes, in which the emitters 104 are positioned while being mounted on the cathode electrode 100. Thus, the emitters 104 each are exposed at a tip end thereof via each of the through-holes of the gate 104.
The emitters 104 of a conical shape may be arranged so as to be spaced from each other at pitches as small as 10 microns or less, so that such emitters as many as tens of thousands to hundreds of thousands may be arranged on the single substrate 102.
Also, the semiconductor processing techniques permit the gate 108 and emitters 104 to be arranged with respect to each other while keeping a distance between the gate 108 and the tip of each of the emitters 104 smaller than a micron, so that application of a voltage V.sub.GE as low as only tens volts between the gate 108 and the emitters 104 permits the emitters to field-emit electrons therefrom. Then, an anode is arranged in a manner to be spaced from and opposite to the gate 108 and has a positive voltage V.sub.A applied thereto, so that electrons field-emitted from the emitters may be captured by the anode.
The FEC thus constructed has such anode current Ia/gate-emitter voltage V.sub.GE characteristics as shown in FIG. 4. More particularly, a gradual increase in voltage V.sub.GE between the gate and the emitters causes the anode current I.sub.A to start to flow through the anode. The voltage V.sub.GE at which flowing of the anode current I.sub.A starts is called a threshold voltage V.sub.TH. This causes an electric field between the gate and the emitters to be about 10.sup.9 V/m, resulting in the emitters starting to emit electrons, so that the anode current I.sub.A starts to flow through the anode. In general, a voltage indicated at V.sub.OP in FIG. 4 which is considerably higher than the threshold voltage V.sub.TH is kept applied between the gate and the emitters, so that the anode current is kept at a level of I.sub.l.
An anode current generated from each of the cone-like emitters is as small as about 1 microampere. Thus, in order to obtain an anode current of a desired increased level, the conventional FEC is so constructed that the emitters are arranged in an array manner.
Arrangement of phosphors on the anode permits electrons field-emitted from the emitters to be impinged on the phosphors when they are captured by the anode, so that the phosphors may emit light. This permits the FEC to be used for an image display device.
Now, a conventional drive circuit for driving an image display device constructed in accordance with the principle described above will be described hereinafter with reference to FIGS. 5 and 6, wherein FIG. 5 shows the drive circuit and FIG. 6 shows waveforms obtained in an operation of the drive circuit.
In the drive circuit shown in FIG. 5, serial gate data are fed to a shift register 50 and then converted into parallel gate data therein, followed by being latched by a latch circuit 51. For this purpose, the shift register 50 has a clock CLK for shift and a clear pulse CLR for clearing the shift register 50 at intervals of a predetermined period input thereto.
The gate data latched by the latch circuit 51 are applied to gate drivers 52-1 to 52-m, respectively. Gate electrodes 53-1 to 53-m each are formed into a stripe-like shape and the gate drivers 52-1 to 52-m successively drive gate electrodes (G1) 53-1 to (Gm) 53-m, respectively.
The data thus applied to the gate electrodes 53-1 to 53-m act as image data. More particularly, the data are used as image data for every cycle T as indicated at G1 to Gm in FIG. 6.
Series cathode data for successively scanning and driving cathode electrodes 57-1 to 57-n are applied to a shift register 54 and then converted into parallel cathode data therein, followed by being latched by a latch circuit 55. For this purpose, the shift register 50 has a clock CLK for shift and a clear pulse CLR for clearing the shift register 54 at intervals of a predetermined period input thereto.
The cathode data latched by the latch circuit 51 are then applied to the cathode drivers 56-1 to 56-n, respectively. Cathode electrodes 56-1 to 56-n are formed into a stripe-like shape and driven by the cathode electrodes (K1) 57-1 to (Kn) 57-n in turn, respectively.
Drive signals respectively applied to the cathode electrodes 57-1 to 57-n each are sequence pulses as indicated K1 to Kn in FIG. 6, have a pulse width T and are generated at a cycle nT.
The gate electrodes 53-1 to 53-m and cathode electrodes 57-1 to 57-n are arranged so as to constitute a matrix in cooperation with each other and emitter arrays E11, E12 - - - E21, E22 - - - Enm are formed on the cathode electrodes 57-1 to 57-n so as to be positioned at intersections between the gate electrodes 53-1 to 53-m and the cathode electrodes 57-1 to 57-n. The emitter arrays E11 to Enm thus arranged constitute picture cells of the image display device. The emitter arrays E11 to E13 in which a predetermined voltage is applied between one of the cathode electrodes 57-1 to 57-n subsequently driven by the drive signals or scan pulse signals K1 to Kn and the gate electrodes 53-1 to 53-m are thus caused to emit electrons, which are then captured by an anode (not shown) arranged above the gate electrodes 53-1 to 53-m in a manner to be spaced therefrom.
The anode has phosphors deposited thereon, so that electrons emitted from the emitter arrays E11 to Enm impinge on the phosphors positionally corresponding to the emitter arrays, resulting in the phosphors emitting light. The gate electrodes 53-1 to 53-m are applied thereto image data, so that light emission or luminescence of the phosphors is carried out depending on the image data, to thereby provide a desired luminous image.
Unfortunately, an image display device formed into a practical display size causes a stray capacitance to be increased to a level as large as 1000 pF, so that rising and falling waveforms on the cathode electrode driven are rendered gentle as shown in FIG. 6. Also, formation of the image display device into the above-described size renders a width T of a pulse for driving the cathode electrode as small as tens microseconds. Addition of gradation to a display of the device causes the width T to be further decreased to a level of hundreds nanoseconds. Thus, the above-described fact that rising and falling of the drive pulse is gentle causes the drive pulse to start rising for driving the next cathode electrode before it adequately falls.
Unfortunately, this leads to leakage luminescence of adjacent picture cells and a failure to increase a speed of a frequency of the drive pulse. Such disadvantages are remarkably caused when the image display device displays a moving image or animation.